(1) Field of the Invention
The present invention relates to a digital recording apparatus that records an encode stream on a digital recording medium such as a DVD-RAM. More specifically, the present invention relates to technology for partitioning an encode stream into a plurality of VOBs and recording the same.
(2) Description of the Related Art
Recently, a DVD recording/playback apparatus having a DVD-RAM with as large as several gigabytes of storage capacity as a recording medium has been put to practical use and has rapidly become widespread. The DVD recording/playback apparatus is based on the Moving Picture Expert Group (MPEG) standard (ISO/IEC13818) and the DVD-VideoRecording Standard (DVD Specifications for Rewritable/Re-recordable Discs., Part3 VIDEO RECORDING) which specify an encoding method for digital audio-visual data.
The MPEG uses a data compression method based on time correlation properties between different pictures, in addition to a conventionally used compression method based on Discrete Cosine Transform (DCT).
The compression method based on the time correlation properties achieves a high compression rate by representing one picture as a difference from corresponding pictures to be reproduced before and after this picture.
Video data is a plurality of sets of still image data (each set of still image data per unit time is hereafter called a video frame), and therefore video data usually contains similar images. As MPEG can provide a high compression rate for video data containing a lot of images similar to one another, MPEG is suitable for compression of video data.
In addition, MPEG can effectively compress data by providing a different compression rate for each image and dynamically assigning a different amount of data to be encoded to each image in accordance with its complexity.
FIG. 1 shows a hierarchical structure of an encode stream which is recorded on a DVD-RAM by means of a DVD recording/playback apparatus.
The encode stream is a set of encoded digital audio-visual data based on the MPEG standard and the DVD-VideoRecording standard.
The MPEG standard defines a Group of Pictures (GOP) hierarchy as follows.
One GOP includes video encode data corresponding to more than one frame and is defined so as to perform a trick play for a video encode stream such as reproduction in a fast-forward mode or in a fast-rewind mode, or a reproduction from a midpoint of the stream. Each picture within a GOP only refers to pictures in the same GOP and therefore a random access within each GOP is possible.
Meanwhile, the DVD-VideoRecording standard defines a Video Object Unit (VOBU) hierarchy and a Video Object (VOB) hierarchy as follows. Here, each encode stream consists of one or more VOBs.
The VOBU includes one or more GOPs and a plurality of pieces of audio data (not shown).
The VOB includes one or more VOBUs which have a piece of encode information in common.
Also, VOB information (VOBI) is recorded for each VOB in the DVD-RAM, in which the encode information is included.
Here, in some procedures of the DVD recording/playback apparatus such as a recording pause and a change in a video resolution during recording, a VOB has to be changed to another VOB to be recorded before and after these procedures.
The recording pause refers to the case where a user presses a pause button during recording to pause the recording and then presses a playback button to cancel the pause and resume the recording, and the case where only commercials are automatically cut in accordance with the difference of sound attributes. In these cases, the apparatus becomes in a standby mode, by which recording can be resumed in a shorter time period than the normal starting mode, and returns to the recording mode.
The change in the video resolution during recording refers to the case where the video resolution is dynamically changed in accordance with the complexity of pictures. This function is for avoiding image degradation by scaling the video resolution down according to the situation, instead of increasing the compression rates for reducing the amount of encoded data, because the latter case causes block noise and mosquito noise which may lead to the significant image degradation.
FIG. 2A shows an example construction of an encode stream when recording is paused and the pause is cancelled.
As shown in this figure, a portion of the encode stream before the recording pause is recorded as VOB#1 and a portion of the encode stream after the cancellation of the pause is recorded as another VOB (i.e., VOB#2).
The following describes the reason why the VOB should be partitioned and recorded before and after the recording pause.
Video Object Information (VOBI) defines an entry “VOB#REC#Time”, and a recording start time for one VOB is recorded in this entry which is present at the start of the VOBI. Accordingly, when different portions of an encode stream before and after a recording pause are recorded as one VOB, these portions of the stream have the same “VOB#REC#Time”. However, this is not desirable as these portions of the streams do not consist of one continuous period. Therefore, the VOB should be partitioned and recorded in this case.
FIG. 2B shows an example construction of an encode stream when the video resolution is changed during recording.
As shown in this figure, a portion of the encode stream before the change in the video resolution is recorded as VOB#11 and a portion of the encode stream after the change in the video resolution is recorded as another VOB (i.e., VOB#12).
The following describes the reason why the VOB should be partitioned and recorded when a video resolution is changed.
The video resolution is necessary for encoding and is recorded for each VOB unit as VOB information. Here, only one video resolution can be present in one VOB and therefore video resolutions for all frames in the same VOB should be identical with one another. Consequently, the VOB should be partitioned when the video resolution is changed.
It is preferable to seamlessly reproduce pictures at the point of the stream where recording is paused or the video resolution is changed during recording. In the above cases, however, different VOBs which should be reproduced continuously may be separately arranged in the tracks at the inner radius and the outer radius of the DVD-RAM, because a normal DVD recording/playback apparatus records encode stream in units of VOBs. In such an arrangement, if the number of the frames included in the preceding VOB is less than a predetermined number that equals to a time enough not to generate a break of the reproduction due to the seeking time of a reproduction head, then reproduction is suspended for a moment (i.e., a break of the reproduction is generated) and therefore seamless reproduction cannot be realized.